Recently, such a lined Hume pipe obtained by covering the inner surface of the Hume pipe with a synthetic resin pipe has been used for a sewer pipe to be embedded in the ground.
The applicant proposed a method of producing such a lined Hume pipe in Japanese Unexamined Patent Publication No. 5049/1984. The prior art method yet has some problems in respect to tight contact between the Hume pipe and the synthetic resin pipe and in respect productivity.
In the prior art method, the synthetic resin pipe is inserted into the Hume pipe, after which it is expanded to come into tight contact with the inner surface of the Hume pipe. However, as this method includes only a single expansion step, a sufficiently tight contact cannot be obtained. Accordingly, in using the lined Hume pipe obtained by the prior art method for the sewer pipe for a long period of time, there is a possibility that the synthetic resin pipe may peel off from the inner surface of the Hume pipe.
Furthermore, in the prior art method, prior to heating and expanding of the synthetic resin pipe inserted into the Hume pipe, one end portion of the synthetic resin pipe is folded outwardly to cover the outer peripheral surface of the spigot of the Hume pipe. However, as the synthetic resin pipe is not fixed in the Hume pipe when it is folded outwardly, the folding operation is very troublesome to carry out. As a result, productivity is reduced.